Hello,
I've installed openbsd 4.8 (X Window System + Gnome) on my Thinkpad T61 - everything works fine except the middle button of the integrated mousepad.
I've read the there is a synaptic-package for BSDs to fix that but I don't know how to install this. Can anyone help me please?

xf86-input-synaptics was enabled for i386 and amd64 architectures on 8 May 2011, which is -current, and will be in 5.0-release. You are running 4.8-release, which is one release back from the most recent: 4.9. See FAQ 5.1 regarding the release cycle and flavors of OpenBSD.

You have three choices:

Custom backport this driver and its dependencies into your 4.8-release environment. The source code is available, and you are welcome to do this on your own. FAQ 5.3 will get you started.

Install -current. You would either upgrade to 4.9-release and upgrade to the most recent -current snapshot, or, cleanly reinstall from the most recent snapshot.

Wait for 5.0-release, scheduled to be published on or about 1 November 2011.

Note, however, that the table at beginning of Section 5.3 states much of what you need to know. While a few people on this site build -current following updating or installing a snapshot, many simply stop here & just run the snapshot itself.

Snapshots may or may not have uncommited code not found in CVS. There is no way to know unless one continues to build from source & compares kernel + userland afterwards.

I suspect you have unknowingly downloaded either different (incompatible) branches or downloaded while the mirror was updating code itself. When downloading source (as root), the following is one correct way to download source to -current:

Here, I have specified both src & xenocara branches to be downloaded sequentially. A value of doing it this way is that it ensures branch synchronization. If you inconsistently specified CVS tags by including -rOPENBSD_4_9 or -rOPENBSD_4_9_BASE as additional cvs(1) parameters, this would also account for mismatched code.

OPENBSD_4_9_BASE represents the CVS tag used to label the source used for the CD sets. Once this branch is tagged, no changes are ever checked into it again. This branch is also known as OpenBSD-release.

OPENBSD_4_9_BASE represents OpenBSD 4.9-stable. If any public patches are made available to OpenBSD -release (mentioned in the errata page...), changes are checked into this branch.

No tag is required to get the head of CVS where all current development is taking place. This is also known as the -current branch.

It is very important to understand the table displayed in Section 5.3.2. If you did not follow this precisely, errors & inconsistencies will arise.

I should have stated: "If you want to build -current, you need to start from the lastest snapshot. If you were attempting to build -stable, you had the wrong source code."

The CVS repository has the complete source code history of OpenBSD, from the latest patches to -current all the way back to the first revisions. Be sure you are using the right tags if you are extracting source code from CVS.

As ocicat stated, but I did not, following the proper process with the right source code is critical.

I downloaded the current CVS directory but it seems not possible to go this way.

Here are the steps you need to take:

Read Section 5.1 to understand the difference between OpenBSD's favors. Read this section for comprehension.

The table in Section 5.3.2 gives you the roadmap you need to follow. You need to download the ISO image of a -current snapshot. On most mirror sites, this can be found at pub/OpenBSD/snapshots/i386.

It appears you have been starting with an ISO image of OpenBSD 4.9-release which is not the beginning point stated in Section 5.3.2's table. Don't improvise. Follow the information found in the FAQ to the letter.